1. Field of the Invention
The present invention is broadly directed to an apparatus and method for keeping the walls of a reactor free of deposits of a solid reaction product formed in a chemical reaction of a precursor gas composition. More particularly, the present invention is directed to an apparatus and method for keeping the walls of a silicon-producing free-space reactor free of undesirable deposits of crusts of silicon.
2. Brief Description of the Prior Art
There are several chemical processes wherein a solid substance is produced by chemical reaction of a suitable precursor gas composition. For example, solid titanium dioxide (an important pigment) is produced on an industrial scale by vapor phase oxidation of titanium halide.
More importantly from the standpoint of the present invention, the process for producing elemental silicon of sufficient purity for semiconductor and solar cell manufacture also involves vapor phase thermal reaction of a suitable precursor gas. As is known in the art, the manufacture of solar cells requires a relatively large amount of silicon. However, because of the severe problems associated with the large-scale production of semiconductor or solar cell grade (hereinafter solar grade) silicon, state-of-the-art processes are, generally speaking, incapable of providing solar grade silicon at a reasonable cost. In fact, the unavailability of solar grade silicon at a reasonable cost represents the principal factor which presently still renders solar cells too expensive for large-scale electrical power generation.
More particularly, state-of-the-art processes for solar grade silicon production involve the vapor phase thermal reaction of a volatile silicon compound such as silicone tetrachloride (SiCl.sub.4), or trichlorosilane (SiHCl.sub.3) with hydrogen (H.sub.2) gas to yield solid silicon and hydrogen chloride (HCl) gas. A presently preferred method for solar grade silicon production utilizes the thermal decomposition of silane (SiH.sub.4) gas to solid silicon (Si) and hydrogen (H.sub.2) gas, in accordance with Equation I. ##EQU1##
In accordance with one well-accepted method of the prior art, the silicon-producing thermal reaction is conducted in a reactor containing a plurality of heated silicon rods whereby the newly-formed silicon epitaxially grows on the crystalline surface of the rods. A disadvantage of this process is that the newly-formed silicon must be periodically removed from the surfaces of the rods.
A relatively recently developed apparatus for production of solar grade silicon makes use of the thermal decomposition reaction shown in Equation I, in a "free space" reactor. More particularly, in this apparatus, silane gas is introduced into a cylindrical reactor, the side walls of which are heated. Ideally, solid silicon forms as a powder which is periodically removed from the reactor through a suitable state-of-the-art "hopper" or like equipment.
Although the "free-space" reactor offers significant promise for relatively economical solar grade silicon production, it presently suffers from a major disadvantage in that the newly-formed silicon tends to deposit in the form of a hard crust on the heated side walls of the reactor. As is well appreciated by those skilled in the art, significant build-up of a silicon crust on the walls of the reactor is incompatible with the continuous and efficient operation of the reactor; primarily because the crust changes the temperature profile of the reactor as it builds up.
Various scraping devices were tried in the prior art in an attempt to continuously or intermittently remove the deposited crust of silicon from the walls of the free-space reactor. For example, a steel ring was continuously or periodically moved up and down in contact with the internal walls of the reactor. However, these prior art "scraping" devices were by-and-large unsuccessful. This was mainly because they tended to contaminate the produced silicon to the extent that the silicon became less suitable or unsuitable for solar cells and semiconductor manufacture.
The present invention is designed to alleviate the above-noted problem and render the operation of the "free-space" silicon-producing reactor continuous and efficient. More broadly, the present invention is applicable to chemical processes wherein a gas phase reaction yields a solid product which tends to build up as an undesirable deposit on the walls of the reactor.
For the state-of-the-art relating to scraping and stirring devices generally used in the chemical, food processing and related industries, reference is made to U.S. Pat. Nos. 4,210,976; 3,550,177; 3,848,289; 3,522,016; 3,001,335; 3,635,901 and 3,835,910. The apparatus disclosed in the above-referenced patents are, broadly speaking, devices wherein rotating scraper blades contact the walls of a reactor, container or crucible to remove solid or pasty material therefrom.